Add declarative custom-engine frontmatter and migrate Crush/OpenCode to shared behavior definitions by Copilot · Pull Request #44465 · github/gh-aw

Copilot · 2026-07-09T04:17:55Z

This introduces a richer engine-definition frontmatter shape for shared/custom agentic engines and uses it to move Crush and OpenCode off bespoke Go implementations onto a common declarative runtime path. The new syntax is expressive enough for CLI-style engines that need install, config, execution, MCP, capability, and auth wiring, including Auggie-like cases.

New engine-definition surface
- Adds engine.experimental, engine.auth, and engine.behaviors to the named engine definition form.
- behaviors captures the runtime contract instead of hard-coding it in Go:
  - capabilities
  - manifest files/path prefixes
  - installation metadata
  - config file generation
  - execution command/args/env
  - MCP config target
- auth provides explicit secret bindings for engines that need engine-specific credentials forwarded into the runtime.
Shared behavior-defined runtime
- Adds a generic BehaviorDefinedEngine that materializes a CodingAgentEngine from frontmatter.
- Reuses the existing universal provider/model routing where applicable, but also supports engine-specific secret/env bindings for nonstandard CLIs.
- Registers imported named engine definitions during compilation so shared workflows can define engines declaratively instead of requiring Go registration.
Crush/OpenCode refactor
- Replaces the bespoke Crush/OpenCode runtime logic with thin wrappers over builtin engine definitions.
- Moves engine-specific behavior into:
  - pkg/workflow/data/engines/crush.md
  - pkg/workflow/data/engines/opencode.md
- Keeps their existing runtime identity while making their install/config/execute behavior data-driven.
Schema and docs
- Extends the workflow schema and frontmatter reference docs to describe the new engine-definition fields.
- Documents the custom-engine shape with behavior blocks and auth bindings so new engines can be added without copy-pasting a dedicated Go implementation.
Coverage
- Adds/updates tests for:
  - schema shape
  - builtin engine definitions
  - imported behavior-defined engines
  - Crush/OpenCode parity through the shared path
- Includes an imported Auggie-style engine example to exercise explicit auth binding and declarative execution.

Example of the new shared engine definition shape:

engine:
  id: auggie
  display-name: Auggie
  experimental: true
  auth:
    - role: session
      secret: AUGMENT_SESSION_AUTH
  behaviors:
    supported-env-var-keys:
      - AUGMENT_SESSION_AUTH
    installation:
      package-manager: npm
      package-name: "@augmentcode/auggie"
      version: "1.0.0"
      step-name: Install Auggie
      binary-name: auggie
      include-node-setup: true
    config-file:
      path: .auggie.json
      step-name: Write Auggie Config
      content: '{"sandbox":"workspace-write"}'
      merge-strategy: json-merge
    execution:
      command-name: auggie
      args: [run]
      step-name: Execute Auggie CLI
      model-env-var: AUGGIE_MODEL
      mcp-config-env-var: AUGGIE_MCP_CONFIG
      write-timestamp: true

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